Power-efficient multi-antenna wireless device

ABSTRACT

A power-efficient wireless device is equipped with multiple (N) antennas. Each antenna is associated with a transmitter unit and a receiver unit. The wireless device also has processing units used to perform various digital processing tasks. The transmitter units, receiver units, and processing units may be selectively enabled or disabled. In an idle state, the wireless device may enable only a subset (e.g., one) of the N receiver units and one or few processing units for signal detection and acquisition. For active communication, the wireless device may enable N tx  transmitter units for data transmission and/or N rx  receiver units for data reception, where 1≧N tx ≧N and 1≧N rx ≧N. The enabled processing units may also be clocked at a lower frequency whenever data is transmitted or received at a data rate lower than the highest data rate. The wireless device may go to sleep whenever possible to conserve power.

I. CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present Application for Patent claims priority to ProvisionalApplication Ser. No. 60/569,018, entitled “Power-Efficient Multi-AntennaWireless Device,” filed May 7, 2004, and assigned to the assignee hereofand hereby expressly incorporated by reference herein.

BACKGROUND

II. Field

The present invention relates generally to an electronics device, andmore specifically to a multi-antenna wireless device.

III. Background

A multiple-input multiple-output (MIMO) communication system employsmultiple (N_(T)) transmit antennas and multiple (N_(R)) receive antennasfor data transmission. A MIMO channel formed by the N_(T) transmitantennas and N_(R) receive antennas may be decomposed into N_(S) spatialchannels, where N_(s)≧min {N_(T), N_(R)}. The N_(S) spatial channels maybe used to transmit data in parallel to achieve higher throughput and/orredundantly to achieve greater reliability.

A multi-antenna wireless device is equipped with multiple antennas thatmay be used for data transmission and/or reception. Each antenna may beassociated with (1) transmit circuitry used to process a baseband outputsignal to generate a radio frequency (RF) output signal suitable fortransmission via a wireless channel and (2) receive circuitry used toprocess an RF input signal to obtain a baseband input signal. Themulti-antenna wireless device also typically has digital circuitry usedto digitally process data for transmission and reception.

Due to the additional complexity associated with MIMO operation, themulti-antenna wireless device may be much more complex than asingle-antenna wireless device in a system that can support bothsingle-antenna and multi-antenna wireless devices. The multi-antennawireless device may thus consume much more power than the single-antennawireless device. The higher power consumption is undesirable, especiallyif the multi-antenna wireless device is a portable unit (e.g., ahandset) that is powered by an internal battery. The higher powerconsumption depletes the available battery resources more quickly, whichthen shortens both the standby time between battery recharges and the ontime for active communication.

There is therefore a need in the art for a multi-antenna wireless devicethat is power efficient.

SUMMARY

A power-efficient multi-antenna wireless device employing various powersaving techniques is described herein. The wireless device is equippedwith multiple (N) antennas. In an embodiment, the multi-antenna wirelessdevice comprises multiple transmitter units operatively coupled to the Nantennas, one transmitter unit for each different set of at least oneantenna among the N antennas. Each transmitter unit processes arespective input baseband signal and provide a radio frequency (RF)output signal. A controller selectively enables one or more transmitterunits, as needed, for transmission. In another embodiment, themulti-antenna wireless device comprises multiple receiver unitsoperatively coupled to the antennas, one receiver unit for eachdifferent set of at least one antenna among the N antennas. Eachreceiver unit processes a respective RF input signal and provide abaseband output signal. The controller selectively enables one or morereceiver units, as needed, for reception. In yet another embodiment, themulti-antenna wireless device comprises at least one processing unit(e.g., a data processor, a spatial processor, a modulator, ademodulator, a detection/acquisition unit, and so on). Each processingunit performs designated processing for transmission or reception via atleast one antenna. Each processing unit is enabled if the processing bythat unit is used for transmission or reception and is disabledotherwise. The controller enables or disables each processing unit, asneeded. The transmitter units, receiver units, and processing units maybe selectively enabled or disabled, for example, by supplying or notsupplying power to these circuit blocks, by enabling or disabling theclocks for the processing units, and so on.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a multi-antenna wireless device.

FIG. 2 shows an embodiment of the multi-antenna wireless device in FIG.1.

FIGS. 3A and 3B show exemplary state diagrams for the wireless device.

FIG. 4 shows a transmission format used by a wireless system.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

FIG. 1 shows a block diagram of a multi-antenna wireless device 100,which is equipped with multiple (N) antennas 122 a through 122 n. Eachantenna 122 is associated with a transceiver unit 120 that includes atransmitter unit (TMTR) and a receiver unit (RCVR).

For data transmission, a transmit (TX) data processor 114 receivestraffic data from a data source 112, processes (e.g., encodes,interleaves, and symbol maps) the traffic data and control data from acontroller 140, and provides data symbols. As used herein, a “datasymbol” is a modulation symbol for data, a “pilot symbol” is amodulation symbol for pilot (which is data that is known a priori byboth a transmitting entity and a receiving entity), a “transmit symbol”is a symbol to be sent from an antenna, and a “received symbol” is asymbol obtained from an antenna. A TX spatial processor 116 performsspatial processing on the data symbols and pilot symbols, if and asappropriate. TX spatial processor 116 provides N_(tx) streams oftransmit symbols, one transmit symbol stream for each antenna to be usedfor data transmission, where 1≧N_(tx)≧N. A modulator 118 performsmodulation on the N_(tx) transmit symbol streams and provides N_(tx)streams of chips, one chip stream for each transmit symbol stream. Eachchip is a complex-value to be transmitted from one antenna in one chipperiod. The modulation performed by modulator 118 is dependent on themodulation technique used by the system. For example, modulator 118performs orthogonal frequency division multiplexing (OFDM) modulation ifthe system utilizes OFDM and performs code division multiplexing (CDM)if the system utilizes CDM. In any case, N_(tx) transmitter unitsselected from among the N transmitter units in transceiver units 120 athrough 120 n receive the N_(tx) chip streams. Each selected transmitterunit conditions its chip stream and generates a corresponding RF outputsignal. The N_(tx) selected transmitter units provide N_(tx) RF outputsignals to N_(tx) antennas associated with these transmitter units. TheN_(tx) RF output signals are transmitted from these N_(tx), antennas.

For data reception, N_(tx) antennas selected from among the N antennas122 a through 122 n receive RF modulated signals transmitted by otherentity(ies), where 1≧N_(rx)≧N. Each selected antenna provides a receivedsignal to the receiver unit in the associated transceiver unit. Eachreceiver unit performs processing complementary to that performed by thetransmitter units at the transmitting entity(ies) and provides a streamof samples. A demodulator 132 receives N_(rx) sample streams from theN_(rx) receiver units for the N_(rx) selected antennas, performsdemodulation on these N_(rx) sample streams, and provides N_(rx) streamsof received symbols. A receive (RX) spatial processor 134 performsreceiver spatial processing (or spatial matched filtering) on the N_(rx)received symbol streams, if and as appropriate, and provides N_(rx)streams of detected data symbols, which are estimates of the datasymbols sent by the transmitting entity(ies). An RX data processor 136processes (e.g., symbol demaps, deinterleaves, and decodes) the detecteddata symbols and provides decoded data to a data sink 138.

Controller 140 controls the operation of various processing units atwireless device 100. Memory unit 142 stores data and program codes usedby controller 140 and other processing units at wireless device 100.

FIG. 1 shows a functional block diagram with various processing unitsfor an exemplary multi-antenna wireless device 100. In general, amulti-antenna wireless device may include any type and any number ofprocessing units. For example, a wireless device may include processingunits for various functions and applications such as video, audio, game,input/output (I/O) interface with external units attached to thewireless device, and so on. The processing units for the multi-antennawireless device may also be implemented in various manners.

FIG. 2 shows a multi-antenna wireless device 100 a, which is oneembodiment of wireless device 100 in FIG. 1. For this embodiment,wireless device 100 a includes a digital section 210, a clock/localoscillator (LO) subsystem 250, and N transceiver units 120 a through 120n coupled to the N antennas 122 a through 122 n, respectively.

Digital section 210 includes various processing units that performdigital processing for data transmission and reception. A processingunit is a circuit block that performs some type of digital processing.This is in contrast to a circuit block that performs no processing(e.g., a memory unit) or a circuit block that performs analog processing(e.g., a transmitter or receiver unit). Within digital section 210, adata processor 214 implements TX data processor 114 and RX dataprocessor 136 in FIG. 1, and a spatial processor 216 implements TXspatial processor 116 and RX spatial processor 134 in FIG. 1. Amodulator 218 and a demodulator 232 implement modulator 118 anddemodulator 132, respectively. A random access memory (RAM) and a readonly memory (ROM) 212 store data and program codes used by variousprocessing units within digital section 210. A detection/acquisitionunit 220 performs processing to detect and acquire signals from othertransmitting entities. A main controller 240 controls the operation ofvarious processing units within wireless device 100 a. A powercontroller 230 performs power management for wireless device 100 a.Controllers 230 and 240 may implement controller 140 in FIG. 1. A mainmemory 242 provides bulk/mass storage for data and codes used by variousprocessing units within wireless device 100 a and may implement datasource 112, data sink 138, and memory 142 in FIG. 1. The circuit blockswithin digital section 210 may be implemented with power-efficientcircuit structures such as latches.

Clock/LO subsystem 250 generates clocks for the processing units withindigital section 210 and LO signals for transceiver units 120 a through120 n. Within subsystem 250, a reference oscillator 252 generates areference clock signal and may be implemented with a temperatecompensated crystal oscillator (TCXO) or some other oscillator design.Voltage controlled oscillators (VCOs) and phase locked loops (PLLs)within a unit 254 receive the reference clock signal and generate an LOsignal (TX_LO) for the transmitter units and an LO signal (RX_LO) forthe receiver units. A clock generator 256 receives a high frequencyclock signal from unit 254 and/or the reference clock signal fromoscillator 252 and generates the clocks for the processing units withindigital section 210. Clock generator 256 may include one or morefrequency dividers. Each divider receives and divides the high frequencyor reference clock signal and provides an output clock. Each processingunit in section 210 may operate based on a set of one or more clocksfrom clock generator 256. The various processing units in section 210may operate based on the same or different sets of clocks.

Each transceiver unit 120 includes a transmitter unit 260 and a receiverunit 280. The transmitter and receiver units may be implemented with asuper-heterodyne architecture or a direct-conversion architecture. Forthe super-heterodyne architecture, frequency conversion between RF andbaseband is performed in multiple stages, e.g., from RF to anintermediate frequency (IF) in one stage, and then from IF to basebandin another stage. For the direct-conversion architecture, frequencyconversion is performed in a single stage, e.g., from RF directly tobaseband. For simplicity, FIG. 2 shows an embodiment of transmitter unit260 and receiver unit 280 using the direct-conversion architecture.

Within transmitter unit 260, a digital-to-analog converter (DAC) 262receives a stream of digital chips from digital section 210, convertsthe chips to analog, and provides an analog baseband output signal. Afilter 264 then filters the baseband output signal to remove undesiredimages generated by the digital-to-analog conversion and provides afiltered baseband signal. An amplifier (Amp) 266 amplifies and buffersthe filtered baseband signal and provides an amplified baseband signal.A mixer 268 modulates the TX_LO signal from unit 254 with the amplifiedbaseband signal and provides an upconverted signal. A power amplifier(PA) 270 amplifies the upconverted signal and provides an RF outputsignal, which is routed through a duplexer (D) 272 and provided toantenna 122.

Within receiver unit 280, a low noise amplifier (LNA) 282 receives an RFinput signal from antenna 122 and via duplexer 272. LNA 282 amplifiesthe RF input signal and provides a conditioned signal having the desiredsignal level. A mixer 284 demodulates the conditioned signal with theRX_LO signal from unit 254 and provides a downconverted signal. A filter286 filters the downconverted signal to pass the desired signalcomponents and remove noise and undesired signals that may be generatedby the downconversion process. An amplifier 288 amplifies and buffersthe filtered signal and provides a baseband input signal. Ananalog-to-digital converter (ADC) 290 converts the baseband input signalto digital and provides a stream of samples to digital section 210.

FIG. 2 shows exemplary designs for the transmitter and receiver units.For these designs, the DAC and ADC are shown as being parts of thetransmitter unit and receiver unit, respectively. In general, thetransmitter and receiver units may each include one or more stages ofamplifier, filter, mixer, and so on, which may be arranged differentlyfrom that shown in FIG. 2. The transmitter and receiver units may or maynot include the DAC and ADC, respectively. The amplifiers may have fixedor variable gains. The transmitter units and the receiver units may beimplemented such that each transceiver unit includes a transmitter unitand a receiver unit, as shown in FIGS. 1 and 2. The transmitter unitsmay also be implemented as one unit or module, and the receiver unitsmay also be implemented as another unit or module. Each transmitter unitmay also be associated with a set of one or more antennas, and eachreceiver unit may also be associated with a set of one or more antennas.

A power source 258 provides power to various circuit blocks withinwireless device 100 a. Power source 258 may include a rechargeablebattery and/or may receive power from an external source via a powerinput, P_(in). Power source 258 may also include switches that receive acontrol signal labeled as C_(p) and selectively provide power to each ofthe circuit blocks receiving power from the power source.

Alternatively, each circuit block may include one or more switches thatreceive a control signal for that circuit block and power on or off thecircuit block. In general, a control signal may include one or multiplesignal lines.

Wireless device 100 a may be designed to consume less power whileproviding the required functionality. This may be achieved by designingthe various circuit blocks within wireless device 100 a such that theymay be individually enabled and disabled.

An analog circuit block may be enabled or disabled by supplying power ornot supplying power to the circuit block, e.g., via one or moreswitches. A digital circuit block may be enabled or disabled by (1)supplying power or not supplying power to the circuit block via one ormore switches and/or (2) enabling or disabling the clocks' to thedigital circuit block. A digital circuit block may be powered on or off(e.g., via a head-switch and/or a foot-switch) to avoid leakage current,which may be large as integrated circuit (IC) fabrication technologycontinues to improve and the size of transistors continues to shrink.

For the embodiment shown in FIG. 2, each receiver unit i may beselectively enabled or disabled based on a respective control signalR_(i), where i=1 . . . N. Similarly, each transmitter unit j may beselectively enabled or disabled based on a respective control signalT_(j), where j=1 . . . N. Wireless device 110 a may enable any number ofreceiver units by properly setting the N control signals for the Nreceiver units and may also enable any number of transmitter units byproperly setting the N control signals for the N transmitter units.

The various circuit blocks within digital section 210 may also bedesigned such that each circuit block (or possibly different sections ofeach circuit block) may be selectively enabled or disabled. For theembodiment shown in FIG. 2, memory unit 212, data processor 214,modulator 218, main memory 242, spatial processor 216, demodulator 232,main controller 240, and detection/acquisition unit 220 are controlledby eight control signals labeled as C₁ through C₈, respectively. Eachcontrol signal may selectively enable or disable all or a portion of theassociated circuit block. Wireless device 100 a may also be designedwith different circuit blocks for the digital section and/or such thatdifferent circuit blocks may be selectively enabled and disabled.

The circuit blocks within clock/LO subsystem 250 may also be selectivelycontrolled for power saving. The VCOs and PLLs in unit 254 may beselectively enabled or disabled based on a control signal labeled as CV.One set of VCO and PLL may be used to generate the TX_LO signal for allN transmitter units, and another set of VCO and PLL may be used togenerate the RX_LO signal for all N receiver units. If only the receiverunits are being used, then the VCO and PLL for the transmitter units maybe disabled to conserve power. Conversely, if only the transmitter unitsare being used, then the VCO and PLL for the receiver units may bedisabled. Clock generator 256 may be controlled based on a controlsignal labeled as C_(k). Clock generator 256 may generate and provide aset of one or more clocks to each processing unit within digital section210. Clock generator 256 may be controlled to (1) disable certain setsof clocks to certain processing units, (2) generate clocks withdifferent (e.g., lower) frequencies for all or certain processing units,and so on. For a digital circuit that is fabricated with complementarymetal oxide semiconductor (CMOS), power consumption is proportional tothe frequency of the clock used for the digital circuit. Lower powerconsumption may be attained for the processing units within digitalsection 210 by reducing the clock frequency for these processing unitswhenever possible.

For clarity, FIG. 2 shows a separate control signal being provided toeach controllable circuit block. A serial bus may also be used tocontrol multiple circuit blocks. For example, each transmitter unit andeach receiver unit may be assigned a unique address and may beindividually enabled or disabled by the serial bus based on its address.

Power controller 230 generates the control signals for the variouscircuit blocks within wireless device 100 a. Power controller 230obtains information indicating the operating state of wireless device100 a (e.g., via user input or main controller 240) and generates thecontrol signals accordingly. Power controller 230 may include timers,state machines, look-up tables, and so on, which may be used to generatethe proper control signals for the various circuit blocks.

With wireless device 100 a designed using modular controllable circuitblocks as described above, reduced power consumption may be achieved byselectively enabling as few circuit blocks as possible to perform therequired tasks and disabling as many circuit blocks as possible toconserve power. The selective enabling/disabling of the circuit blocksmay be performed as described below.

Wireless device 100 a may support a number of operating modes. Table 1lists some of the supported operating modes and their shortdescriptions. TABLE 1 Operating Mode Description Single-input (SI)Reception of one RF input signal from one antenna. Multiple-input (MI)Reception of multiple RF input signals from multiple antennas.Single-output (SO) Transmission of one RF output signal from oneantenna. Multiple-output (MO) Transmission of multiple RF output signalsfrom multiple antennas.

The SI and MI modes are receive-only modes, and the SO and MO modes aretransmit-only modes. Additional operating modes may be formed withdifferent combinations of the four operating modes shown in Table 1. Forexample, a SISO mode supports transmission of one RF output signal andreception of one RF input signal, a MISO mode supports transmission ofone RF output signal and reception of multiple RF input signals, a SIMOmode supports transmission of multiple RF output signals and receptionof one RF input signal, and a MIMO mode supports transmission ofmultiple RF output signals and reception of multiple RF input signals.The SISO, SIMO, MISO, and MIMO modes are transmit-receive modes thatsupport both data transmission and reception. The same or differentantennas may be used for data transmission and reception when fewer thanN antennas are selected for use. A “standby” mode may also be definedwhereby data is neither being transmitted nor received.

For each operating mode, wireless device 100 a may enable only thetransmitter units and only the receiver units that are required for thatoperating mode. For example, all transmitter units may be disabled forthe receive-only modes, all receiver units may be disabled for thetransmit-only modes, and all transmitter and receiver units may bedisabled for the standby mode. Wireless device 100 a may also disablethe unused transmitter units and receiver units when fewer than N unitsare selected for use.

Wireless device 100 a may also disable certain processing units withindigital section 210 for some operating modes. For example, modulator 218may be disabled for the receive-only modes, demodulator 232 may bedisabled for the transmit-only modes, and spatial processor 216 may bedisabled for the SI and SO modes. A transmit portion of data processor214 (e.g., for encoding, interleaving, and symbol mapping) maybedisabled for the receive-only modes, and a receive portion of dataprocessor 214 (e.g., for decoding, deinterleaving, and symbol demapping)may be disabled for the transmit-only modes.

Wireless device 100 a may also adjust the frequency of the clocks forthe processing units within digital section 210 based on the data ratesfor data transmission and reception. The system may support a range ofdata rates, and the difference between the lowest data rate, R_(min),and the highest data rate, R_(max), may be large. Wireless device 100 amay be designed with the capability to transmit and receive data at thehighest data rate. This is typically achieved by designing theprocessing units with sufficient processing capabilities and operatingthe processing units at a specified maximum clock frequency, f_(max).However, data is typically not sent and received at the highest datarate all of the time. When a data rate lower than R_(max) is used, itmay be possible to operate the processing units at a clock frequencythat is lower than f_(max) to reduce power consumption while stillcompleting all of the required processing tasks in a timely manner. Theclock frequency for the processing units may be a function of data rate,so that different clock frequencies may be used for different datarates. Power controller 230 may determine the lowest possible clockfrequency that may be used for the current data rate and provide theappropriate control signal to clock generator 256. Clock generator 256may then adjust its divider circuitry based on the control signal togenerate clocks at the desired frequency.

The same or different data rates may be used for data transmission andreception. The same or different clock frequencies may be used for theprocessing units used for data transmission and reception, depending onthe transmit and receive data rates. Modulator 218 and demodulator 232may operate at the same or different clock frequencies. The transmit andreceive portions of data processor 214 may also operate at the same ordifferent clock frequencies. Spatial processor 216 may containprocessing engines (e.g., multipliers) that may be used for both datatransmission and reception. The clock frequency for these engines may beappropriately selected based on the transmit and receive data rates.

Wireless device 100 a may also be designed to operate in a number ofstates. Each state may be associated with different processingcapabilities and different tasks.

FIG. 3A shows an exemplary state diagram 300 for wireless device 100 a.This state diagram includes three states—an Idle state 310, a Sleepstate 320, and a Communication state 330. Each of the three states mayinclude a number of substates.

FIG. 3B shows an exemplary state diagram for Idle state 310, whichincludes two substates—a Monitoring substate 312 and an Access substate314.

In the Monitoring substate, wireless device 100 a detects for thepresence of signals from other entities. The detection may be performedbased on a pilot transmitted by each entity. If any signal is detected,then wireless device 100 a attempts to acquire the frequency and timingof the signal and to recover overhead information and signaling messagesfrom the signal. If the signal is acquired and the signaling messagesindicate that a data transmission requested, then wireless device 100 atransitions to the Access substate. Wireless device 100 a does nottransition to the Access substate if (1) no signal is detected or (2) asignal is detected but either acquisition fails or the signalingmessages do not indicate that a data transmission is requested. Wirelessdevice 100 a may remain in the Monitoring substate and continuallyperform detection and acquisition. Alternatively, wireless device 100 amay perform detection and acquisition periodically or sporadically incertain time windows, which are called awake periods. Wireless device100 a may transition from the Monitoring substate to the Sleep state inthe time between the awake periods to conserve power, as describedbelow.

In the Access substate, wireless device 100 a attempts to establish acommunication session with another entity, which is called a “target”entity. Wireless device 100 a attempts to access the system if thetarget entity is an access point in the system and attempts to establishpeer-to-peer communication if the target entity is another wirelessdevice. Wireless device 100 a may exchange short messages with thetarget entity to configure both entities for data transmission and/orreception, request resources, and so on. Wireless device 100 atransitions to the Communication state if access is successful and acommunication session is opened. Wireless device 100 a returns to theMonitoring substate if access fails.

The wireless device may spend a large percentage of time in the Idlestate. In the Idle state, the wireless device may enable only a subset(e.g., one) of the N receiver units and only the processing unit(s) usedfor signal detection and acquisition. The wireless device may disableall other receiver units, all N transmitter units, and all processingunits that are not needed for signal detection and acquisition.

In the Communication state, wireless device 100 a may transmit data toand/or receive data from the target entity. Wireless device 100 a or thetarget entity may terminate the communication session, at which timewireless device 100 a returns to the Monitoring substate within the Idlestate.

In the Sleep state, wireless device 100 a may shut down as many circuitblocks as possible to conserve power. Wireless device 100 a does nottransmit or receive data while in the Sleep state. Wireless device 100 amay maintain a timer to determine when to wake up from the Sleep stateand may monitor for certain interrupts (e.g., user input) that cantrigger an immediate transition to the Idle state.

Wireless device 100 a may operate in any one of a number of “detection”modes that do not require continuous detection/acquisition. For example,if the system transmits pilots, overhead information, and signalingmessages at known times, then wireless device 100 a may operate in a“slotted” mode whereby it performs detection and acquisition only duringthese known times and goes to sleep during the remaining times, asdescribed below. Wireless device 100 a may also operate in a“power-save” mode whereby it goes to sleep indefinitely to conservepower and wakes up only if the user initiates a data transmission orsome other event triggers a transition out of sleep.

FIGS. 3A and 3B show exemplary state diagrams for wireless device 100 a.In general, a multi-antenna wireless device may operate in any number ofstates, and each state may include any number of substates.

Wireless device 100 a may enable different circuit blocks in differentstates.

Some possible operating scenarios for the three states in FIG. 3A aredescribed below.

In the Idle state, wireless device 100 a may enable just a subset of theN receiver units and may disable all remaining receiver units as well asall N transmitter units. A transmitting entity typically transmits datain the most robust manner and at the lowest data rate when transmittingto an unknown receiving entity. Receive diversity and detectionperformance typically improve if the receiving entity uses more antennasto receive the transmission from the transmitting entity. However, it isnormally not necessary to use all N antennas to detect for the presenceof a signal or to acquire the signal. Power may be conserved by enablingonly a limited number (e.g., one) of the receiver units and disablingthe remaining receiver units.

In the Idle state, wireless device 100 a may enable the desired numberof receiver units via the control signals R_(i) for these receiverunits. For example, wireless device 100 a may operate in the SI mode forthe Idle state and enable only one receiver unit. Wireless device 100 amay also use different capabilities for different processing tasks. Forexample, wireless device 100 a may use one receiver unit for detectionand acquisition of pilots from other entities and may use multiplereceiver units to receive overhead information and signaling messages ifpilots are detected.

In the Idle state, many of the circuit blocks in digital sections 210that are used to process data for transmission and reception may also bedisabled to conserve power.

For example, data processor 214, spatial processor 216, modulator 218,and demodulator 232 may be disabled in the Idle state.Detection/acquisition unit 220 may perform the necessary processingtasks to detect for signals and to acquire detected signals. Unit 220may include circuitry to search for pilots, measure received pilotpower, recover overhead information and signaling messages, and so on.

All or a portion of demodulator 232 and all or a portion of dataprocessor 214 may be enabled in the Idle state, if and as necessary. Ifthe system utilizes OFDM, then demodulator 232 may include one or morefast Fourier transform (FFT) engines used for OFDM demodulation. Aportion of demodulator 232 may be enabled to perform OFDM demodulationfor all of the receiver unit(s) enabled in the Idle state. Dataprocessor 214 may include encoders, decoders, and so on, which are usedfor data transmission and reception. A portion of data processor 214(e.g., the decoder for overhead information and signaling messages) maybe enabled, e.g., only if a signal has been detected and acquired.

Spatial processor 216 is used for spatial processing to transmit datavia multiple antennas and for spatial processing to receive data viamultiple antennas. If only a single receiver unit is selected for use inthe Idle state, then spatial processor 216 may be disabled during theentire time in the Idle state. Even if a few receiver units are selectedfor use in the Idle state, the receiver spatial processing for thesereceiver units may be relatively simple and may be performed by unit220. Alternatively, spatial processor 216 may be enabled as necessary toperform receiver spatial processing for the few receiver units that areenabled in the Idle state.

In the Communication state, data may be transmitted continuously orintermittently, and data may also be received continuously orintermittently. At any given moment, wireless device 100 a may operatein (1) a transmit-only mode if data is being transmitted but notreceived, (2) a receive-only mode if data is being received but nottransmitted, (3) a transmit-receive mode if data is being transmittedand received, or (4) the standby mode if data is neither beingtransmitted nor received. Furthermore, wireless device 100 a may useone, few, or all N transmitter units for data transmission and may useone, few, or all N receiver units for data reception. This depends onvarious factors such as, for example, the manner in which data istransmitted and received, the capability of the other entity with whichwireless device 100 a is in communication, and so on. For example, atransmission mode may use a specific number of (e.g., two) antennas totransmit data. As another example, the channel condition may be suchthat it is better to use fewer than N antennas for transmission and todistribute the total transmit power over fewer antennas so that moretransmit power may be used for each enabled antenna (subject to themaximum output transmit power limit of the power amplifier and/or theantenna). Fewer than N antennas may also be used to receive a datatransmission, e.g., if data is transmitted at a low data rate, if theother entity is equipped with one or a few antennas, and so on. In anycase, wireless device 100 a may enable only the transmitter units andonly the receiver units, if any, that are required for the currentoperating mode and may disable all other transmitter and receiver units.

In general, for communication with another entity, the wireless devicemay utilize N_(tx) of the N transmitter units for data transmissionand/or N_(rx) of the N receiver units for data reception, where1≦N_(tx)≦N, 1≦N_(rx)≦N, and N_(tx) may or may not be equal to N_(rx). Atany given moment, all N transmitter units and all N receiver units maynot be required for communication for various reasons. In this case, thewireless device may disable transmitter and receiver units as well asprocessing units that are not used for communication.

Data may be transmitted and received at the highest data rates possiblein order to conserve power and obtain other benefits. The highesttransmit data rate may be achieved by using all N antennas for datatransmission and using the maximum transmit power available for wirelessdevice 100 a. The highest receive data rate may be achieved by using allN antennas for data reception. For a given amount of data to be sent orreceived, a higher data rate allows for transmission or reception of thedata in a shorter amount of time, which in turn allows the transmitterunits and receiver units to be enabled for a shorter amount of time.Operating the processing units within digital section 210 at a higherclock frequency but for a shorter amount of time may reduce powerconsumption.

For both the Idle and Communication states, the processing units withindigital section 210 may be clocked at a lower frequency wheneverpossible in order to reduce power consumption. In the Idle state,signaling messages may be sent at the lowest data rate, R_(min), andwireless device 100 a may use a low clock frequency to process andrecover these messages. In the Communication state, data may be sent andreceived at variable data rates that may be determined based on variousfactors such as the channel condition, the available system resources atthe transmitting and receiving entities, and so on. When data istransmitted and received at data rates lower than the highest data rate,R_(max) the clock frequency for the processing units within digitalsection 210 may be reduced to reduce power consumption, as describedabove.

In the Sleep state, wireless device 100 a may disable as many circuitblocks as possible in order to conserve power. For example, wirelessdevice 100 a may disable all N transmitter units, all N receiver units,all or most of the clock/LO subsystem, and most of the circuit blocks indigital section 210. Power controller 230 may provide the proper controlsignal to power source 258 and/or the circuit blocks to power down thevarious circuit blocks. Power controller 230 may also provide the propercontrol signal to clock generator 256, which disables the clocks to thedisabled processing units within digital section 210. Power controller230 may maintain a timer that counts down the amount of time spent inthe Sleep state and informs the power controller when the timer expires.Power controller 230 may also include circuitry used to detect forinterrupts (e.g., user inputs) that can trigger an immediate transitionout of the Sleep state.

The system may include one or more access points that communicate withwireless devices under their coverage area. The system may also bedesigned such that the access points transmit pilots, overheadinformation, and signaling messages at designated times instead ofcontinuously. Additional power saving may be realized for wirelessdevices operating in such a system.

FIG. 4 shows a timing diagram for an exemplary transmission format usedby the system. For this transmission format, each access point transmitsa pilot, overhead information (OH Info), and signaling messagesperiodically at designated times. For example, the pilot, overheadinformation, and signaling messages may be transmitted starting at fixedlocations in each frame, which has a fixed time duration (e.g., 2 msec).The pilot may be used for signal detection, acquisition, channelestimation, and possibly other purposes. The overhead information mayinclude various system parameters such as, e.g., the data rate used forthe signaling messages, the duration of the messages, and so on. Thesignaling messages may be broadcast messages intended to be received byall wireless devices, multicast messages intended to be received by aspecific group of wireless devices, and/or unicast messages intended tobe received by a specific wireless devices.

Wireless device 100 a may detect for pilots transmitted by the accesspoints in the system and attempt to acquire the timing and frequency ofeach detected pilot. Wireless device 100 a may then attempt to recoverthe overhead information for each access point that has beensuccessfully acquired and to decode and recover signaling messages usingthe recovered overhead information.

If wireless device 100 a is not in communication with any access pointor another wireless device, then it may operate in the slotted mode andperiodically detect for signals and messages from other entities.Wireless device 100 a may wake up prior to the time that pilots areexpected to be transmitted and perform monitoring tasks (e.g., detectand process pilots, process overhead information and signaling messages,and so on). Wireless device 100 a may then go to sleep after all of themonitoring tasks are completed and no message requires wireless device100 a to remain awake. Wireless device 100 a may sleep until the startof the next pilot transmission or some other time instant.

For simplicity, FIG. 4 shows one pilot transmission for each frame. Theaccess point may transmit multiple pilots of different types in eachframe, and each pilot may be used for different purposes. For example, a“beacon” pilot may be sent in a manner to facilitate detection andtiming/frequency acquisition, a “MIMO” pilot may be sent in a manner tofacilitate channel estimation for a MIMO channel, and so on. Wirelessdevice 100 a may utilize one receiver unit to detect for the beaconpilot and may utilize all N receiver units to process the MIMO pilot.

The multi-antenna wireless device and the power saving techniquesdescribed herein may be implemented by various means. For example, thecircuit blocks in the digital section of the wireless device may beimplemented with one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, micro-controllers,microprocessors, other electronic units designed to perform thefunctions described herein, or a combination thereof. The circuit blocksin the analog section of the wireless device may be implemented with oneor more RF integrated circuits (RFICs), discrete components, and so on.

The power saving techniques may be implemented with hardware, software,or a combination thereof. The selective enabling and disabling of thevarious circuit blocks within the wireless device may be performed by ahardware unit (e.g., a micro-controller, a state machine, and so on).The selective enabling and disabling of the circuit blocks may also beperformed by software codes executed on a processor. The software codesmay be stored in a memory unit (e.g., memory unit 242 in FIG. 2) andexecuted by a processor (e.g., power controller 230). The memory unitmay be implemented within the processor or external to the processor, inwhich case it can be communicatively coupled to the processor viavarious means as is known in the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A multi-antenna wireless device comprising: a plurality of (N) transmitter units operatively coupled to a plurality of antennas, one transmitter unit for each set of at least one antenna from among the plurality of antennas, each transmitter unit operable to process a respective input signal and provide an output signal; and a controller to selectively enable or disable at least one of the N transmitter units.
 2. The wireless device of claim 1, wherein the controller selects one of a plurality of operating modes for the wireless device, each operating mode utilizing a different combination of the N transmitter units.
 3. The wireless device of claim 2, wherein the controller enables one of the N transmitter units and disables remaining ones of the N transmitter units if a mode supporting transmission of a single output signal is selected, and wherein the enabled transmitter unit generates the single output signal.
 4. The wireless device of claim 2, wherein the controller enables multiple ones of the N transmitter units and disables remaining ones, if any, of the N transmitter units if a mode supporting transmission of multiple output signals is selected, and wherein the enabled transmitter units generate the multiple output signals.
 5. The wireless device of claim 1, wherein the controller disables the N transmitter units when data is not transmitted.
 6. The wireless device of claim 1, further comprising: a power source operative to cut off power to each transmitter unit that is disabled.
 7. An apparatus comprising: a plurality of (N) transmitter units operatively coupled to a plurality of antennas, one transmitter unit for each set of at least one antenna from among the plurality of antennas, each transmitter unit operable to process a respective input signal and provide an output signal; and means for selectively enabling or disabling at least one of the N transmitter units.
 8. The apparatus of claim 7, wherein the means for selectively enabling or disabling comprises means for enabling one of the N transmitter units and disabling remaining ones of the N transmitter units if a mode supporting transmission of a single output signal is selected, and wherein the enabled transmitter unit generates the single output signal.
 9. The apparatus of claim 7, wherein the means for selectively enabling or disabling comprises means for enabling multiple ones of the N transmitter units and disabling remaining ones, if any, of the N transmitter units if a mode supporting transmission of multiple output signals is selected, and wherein the enabled transmitter units generate the multiple output signals.
 10. The apparatus of claim 7, further comprising: means for cutting off power to each transmitter unit that is disabled.
 11. A multi-antenna wireless device comprising: a plurality of (N) receiver units operatively coupled to a plurality of antennas, one receiver unit for each set of at least one antenna from among the plurality of antennas, each receiver unit operable to process a respective input signal and provide an output signal; and a controller to selectively enable or disable at least one of the N receiver units.
 12. The wireless device of claim 11, wherein the controller selects one of a plurality of operating modes for the wireless device, each operating mode utilizing a different combination of the N receiver units.
 13. The wireless device of claim 12, wherein the controller enables one of the N receiver units and disables remaining ones of the N receiver units if a mode supporting reception of a single input signal is selected, and wherein the enabled receiver unit processes the single input signal.
 14. The wireless device of claim 12, wherein the controller enables multiple ones of the N receiver units and disables remaining ones, if any, of the N receiver units if a mode supporting reception of multiple input signals is selected, and wherein the enabled receiver units process the multiple input signals.
 15. The wireless device of claim 11, wherein the controller disables the N receiver units when data is not received.
 16. The wireless device of claim 11, further comprising: a power source operative to cut off power to each receiver unit that is disabled.
 17. A multi-antenna wireless device comprising: at least one processing unit, each processing unit operable to perform designated processing for data transmission or reception via at least one of a plurality of antennas, each processing unit being enabled if the designated processing by the processing unit is used for data transmission or reception and disabled otherwise; and a controller to enable or disable each of the at least one processing unit.
 18. The wireless device of claim 17, wherein the at least one processing unit comprises a detection and acquisition unit to perform signal detection and acquisition, wherein the detection and acquisition unit is enabled if the wireless device is monitoring for signals from other entities and disabled otherwise.
 19. The wireless device of claim 17, wherein the at least one processing unit comprises a spatial processor to perform spatial processing for data transmission and reception, wherein the spatial processor is enabled to perform spatial processing if multiple ones of the plurality of antennas are used for data transmission or reception and disabled otherwise.
 20. The wireless device of claim 17, wherein the at least one processing unit comprises a data processor to process data for transmission and reception, wherein the data processor is enabled if data processing is performed for transmission or reception and disabled otherwise.
 21. The wireless device of claim 17, wherein the at least one processing unit comprises a modulator to perform modulation for data transmission, wherein the modulator is enabled if data is being transmitted and disabled otherwise.
 22. The wireless device of claim 17, wherein the at least one processing unit comprises a demodulator to perform demodulation for data reception, wherein the demodulator is enabled if data is being received and disabled otherwise.
 23. The wireless device of claim 17, further comprising: a power source operative to cut off power to each processing unit that is disabled.
 24. The wireless device of claim 17, further comprising: a clock generator to generate clocks for processing units that are enabled and to disable clocks to processing units that are disabled.
 25. The wireless device of claim 17, further comprising: a clock generator to generate clocks for the at least one processing unit, the clocks having a variable frequency determined by data rates used for data transmission or reception.
 26. The wireless device of claim 17, wherein the controller disables the at least one processing unit when data is not transmitted or received. 